1. Field of the Invention
The present invention relates to detection of Salmonella antigens and to a method of treating an antibody to improve specificity of the test.
2. Description of the Prior Art
An immunofluorescent procedure for detection of antigens such as Salmonella in foods and animal by-products is in general use (Silliker, J. H. et al J. Food Sci 31: 240-244, 1966). The fluorescent antibody technique is supposed to be specific. However, non-Salmonella antigens of similar rod-like morphology sometimes react with the fluorescent antibody to give a false positive reaction. It has been determined that Lactobacillus plantarum strain GT, a harmless rod-like antigen, masquerades as Salmonella by binding to the immunoglobulin G (IgG) and gives false positive reactions in the Salmonella fluorescent antibody detection procedure. It has further been determined that the non-Salmonella Lactobacillus binds non-specifically to the non-antibody Fc portion of the IgG molecule.
Antibodies belong to the class of proteins called immunoglobulins. Immunoglobulin gamma (designated IgG) is the predominant antibody in the blood of most higher vertebrates and has been the most intensively studied of the five types of immunoglobulins. It has a molecular weight of about 160,000, which indicates that its molecule consists of some 23,000 atoms.
IgG is made up of four polypeptide chains, each consisting of amino acid units joined by peptide bonds. The four chains are paired so that the molecule consists of two identical halves, each with one "heavy", or long, chain and one "light", or short, chain. The two chains of each pair are cross-linked by covalent bonds between the sulfur atoms of the amino acid cystine. If these disulfide bonds are split, the heavy and light chains will remain bound to one another by noncovalent interactions.
IgG molecules are composed of three parts of about equal size. Two of them are identical and are designated Fab, for "fragment, antigen-binding". The Fab fragments of an IgG molecule each have a combining site. It is because the intact antibody molecule has two such sites that it is able to cross-link antigenic materials into inactive complexes. The third fragment is designated Fc because it crystallizes readily. It does not bind antigen, but it has other important biological activities.
In the currently accepted model of IgG, the heavy and light chains are arranged in the shape of a Y. According to this model, the Fc fragment is the stem of the Y and consists of the lower half of the two heavy polypeptide chains, which are joined together by one interchain disulfide bridge or more. The two Fab fragments are the prongs of the Y, and each consists of one entire light chain and the rest of the heavy chain, with an antigen-combining site of identical specificity at the far end.